#include<stdio.h>
int main()
{
    float x,y;
    scanf("%f",&x);
    if(x<=15)
    {
        y=4*x/3;
        printf("%.2f",y);
    }
    if(x>15)
    {
        y=2.5*x-10.5;
        printf("%.2f",y);
    }
}
